1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a fixed-focus lens.
2. Description of Related Art
Generally, a longer projection distance is required if a projector needs to project an image onto a larger screen. Contrarily, a special wide-angle lens is usually required if the image needs to be projected onto the larger screen from a shorter projection distance.
How to reduce image aberration is one of the major subjects in the design of wide-angle lenses. Thus, various techniques (for example, the U.S. Pat. No. 6,621,645,U.S. Pat. No. 6,560,041,U.S. Pat. No. 6,999,247,U.S. Pat. No. 6,542,316,U.S. Pat. No. 6,885,506, U.S. Pat. No. 7,184,219, U.S. Pat. No. 7,126,767, U.S. Pat. No. 7,123,426, and U.S. Pat. No. 7,173,777) have been provided to reduce image aberration in a wide-angle lens.
In the U.S. Pat. No. 6,621,645 and U.S. Pat. No. 6,560,041, some lenses in a wide-angle lens are implemented with aspherical lenses. However, in the U.S. Pat. No. 6,621,645, at least one molding glass is adopted, and in the U.S. Pat. No. 6,560,041, at least three aspherical lenses are adopted. As a result, the production cost of the wide-angle lens is increased, and the assembly process thereof is made more complicated. Contrarily, if fewer aspherical lenses are adopted and at the same time, the image aberration is improved, both the total length of the wide-angle lens and the volume of the projection system are increased. For example, the total lengths of both lenses in the U.S. Pat. No. 6,999,247 and U.S. Pat. No. 6,542,316 are greater than 150 mm.
On the other hand, more lenses may also be disposed to reduce image aberration. For example, more than fourteen lenses are respectively adopted in the U.S. Pat. No. 6,885,506 and U.S. Pat. No. 7,184,219. However, this may complicate the assembly process and increase the production cost of the wide-angle lens. Thereby, a lens with both lower production cost and higher imaging quality is to be provided.